It is commonplace in semiconductor devices to use adjacent regions of opposite conductivity type, as for example the base and emitter of a bipolar transistor or the source and gate of a field effect transistor. Where the semiconductor devices are to operate with high efficiency, at high frequency, and/or with fast switching speeds, it is important to place the contacts to these oppositely doped regions as close as possible to the intervening junction so that lateral voltage drops are minimized. This is particularly true where at least one of the adjacent semiconductor regions has comparatively high resistivity. The emitter-base of a bipolar transistor is an example of such a situation.
One method for providing contacts to adjacent device regions on silicon has been to use an intermediate poly-silicon region or layer between the metallic interconnections and the device regions. For example, with a bipolar transistor a poly-silicon layer is often used as an intermediate contact to the base or emitter region. While poly-silicon contacts have resulted in improved performance, the prior art arrangements and methods have significant drawbacks that limit their usefulness. For example, with the prior art techniques the poly-layer used for a base contact must generally have an initial thickness which is much greater than is otherwise desired. This is because the poly-layer which forms the base contact in the prior art is often exposed to numerous etching and oxidation steps during fabrication. The extra thickness must be provided when the poly-layer is formed to make up for the portions of the poly-layer consumed during processing. This extra thickness is not desirable since it makes fabrication more difficult and degrades performance of the device or circuit. Further, with prior art structures the poly-contacts are often not as close to the junction as is desired and regions of relatively high resistivity material or equivalent can still remain between, for example, the poly-base contact and the base-emitter junction of a bipolar transistor. Thus, a need continues to exist for device contacting structures and device fabrication methods which provide closely spaced contacts to regions of opposite conductivity type, which provide enhanced conductivity regions between the contacts and the junction, and which avoid many drawbacks of the prior art.
Accordingly, it is an object of the present invention to provide an improved structure for making contacts to adjacent semiconductor regions.
It is an additional object of the present invention to provide an improved method for producing contacts to adjacent semiconductor regions.
It is a further object of the present invention to provide the above improved structure and method wherein the contacts are self-aligned.
It is an additional object of the present invention to provide an improved structure and method for fabricating poly-contacts to adjacent semiconductor regions wherein the poly-contacts to one or more of the semiconductor regions are protected from etching and oxidation during device fabrication.
It is a further object of the present invention to provide an improved means and method for semiconductor devices wherein adjacent device regions and/or the contacts to the adjacent regions are self-aligned and their separation precisely controlled.
It is an additional object of the present invention to provide a means and method for including enhanced conductivity regions between the contacts and the junction so as to reduce the series resistance within the device.
As used herein the prefix poly- and the word poly are generally intended to refer to polycrystalline or amorphous materials.